The practice is known (West German Pat. No. 1.287.484) of measuring the thickness of the silver before the draft zone and according to the measured values to adjust in the axial direction pins carried on a disc defining a continuous circulating store. The positions of the pins relative to the disc are sensed and corresponding electrical control pulses are generated. The pulses are fed to a control device which alters the ratio of transmission of a taper-roller gearing and thereby the draft ratio of the draft zone in such a way that the silver leaving the draft zone exhibits essentially constant thickness. Such a store provided with pins is, however, suitable only for relatively low draft speeds. But with the high speeds utilized today, satisfactory setting of the pins is not possible since rapid adjustment of the pins leads to throwing, so that no uniform sliver thickness can be achieved.
For higher operating speeds the practice is, however, known of inserting dimensional analogue voltages determined by the scanning, unchanged into an electrical store and recalling them with a phase shift for the control of the separate drive of the draft stage (West German Pat. O/S No. 2 331 217). But such a method is inaccurate since dimensional quantities cannot be stored as unaltered values for long, causing faulty regulation.
As a result of partial discharge of capacitors used in storage devices because of leakage currents in the case of long stoppage times of the machine and by ageing and deterioration, accurate regulation cannot be guaranteed in the case of the known method.